1. Field of the Invention
The present invention relates to an optical apparatus for measuring roughness of a surface of a subject.
2. Discussion of the Prior Art
In an example of a presently available optical surface roughness measuring apparatus, a subject is mounted on a rotary table rotatably supported by an air bearing, and areas of the surface of the subject are irradiated by two linearly polarized beams which are produced by a common laser source. Described more particularly, the two linearly polarized laser beams have mutually perpendicular polarization planes and different frequencies fs and fp, and are referred to as S-type and P-type linearly polarized beams. These S-type and P-type beams are converged by a Wollaston polarizing prism, at a first and a second point of convergence on the subject surface, respectively. The first convergence point is aligned with the axis of rotation of the subject, and the second convergence point is located adjacent to the first convergence point. The S-type and P-type linearly polarized laser beams reflected by the first and second convergence points on the subject surface have respective frequencies of fs+.DELTA.fs, and fp+ .DELTA.fp. With these reflected laser beams received by suitable photosensor means, a measuring beat signal having a beat frequency of fs-fp+.DELTA.fs-.DELTA.fp is obtained. An amount of displacement Zp of the subject surface at the second convergence point is obtained by integrating phase shift amounts of the measuring beat signal according to the following equation, provided that .DELTA.fs is substantially zero, that is, the surface roughness at the first convergence point is zero (namely, a portion of the subject surface at the first convergence point or axis of rotation of the subject is mirror-smooth): EQU Zp=.lambda./2.intg..DELTA.fp dt=.lambda./2.intg.(fd-fb) dt=.lambda./2 (Cd-Cb)
where,
fd: Measuring beat signal PA1 fb: Reference beat signal PA1 Cd: Count of pulses of the measuring beat signal PA1 Cb: Count of pulses of the reference beat signal
As described above, the above-described known optical surface roughness measuring apparatus requires the subject surface to have complete mirror-smoothness in an extremely small area around the axis of rotation of the subject, which mirror-smooth area is irradiated by the S-type linearly polarized beam. Usually, the subject to be measured does not have such a mirror-smooth area. Further, the use of a member having such a mirror-smooth surface area around the axis of rotation of the subject, makes it difficult or impossible to effect optical measurement of the subject surface.
Another disadvantage of the known optical surface roughness measuring apparatus arises from the use of a Wollaston polarizing prism for separating the S-type and P-type linearly polarized beams so that the two beams are converged at the first and second spaced-apart convergence points. Generally, the Wollaston polarizing prism is not suitable for controlling the optical paths of the S-type and P-type linearly polarized beams such that the points of convergence of the two beams are spaced apart from each other by only a small distance, for example, by a distance of a few microns. Accordingly, a precession movement or oscillation of the subject is likely to increase a measuring error of the apparatus.
Furthermore, the known apparatus using a rotary table permits only the measurement of roughness of a circumferential portion of the subject surface, and therefore makes it difficult to process optical roughness data for providing a three-dimensional display of the measured surface roughness of the subject.